LED Answered

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Well, that depends on what sort of LED project you want to do. (in particular, the color) Voltages are fairly standard by color, with red being ~1.5V, green being 3-4V, and blue generally in the 4V range.
But no need to limit yourself to the standard colors, there are also nonstandard colors, like orange, purple, white. You can even get infrared and ultraviolet leds, if you're feeling adventurous.
Hope that helps,
Purduecer